Charge induction



March 2, 1965 R. w. GUNDLACH 3,172,024

CHARGE INDUCTIION Filed March 11'. 1960 2 Sheets-Sheet 2 Q OWER SU 76-"EXPOSURE STATION O f a? T L 77 g I ERA s5 78 a0 PROCESSING I 82 mPROCESS/N6 ROBERT W. GUNDLACH a gbkfl QQQ A TTORNE Y United StatesPatent 3,172,024 CHARGE INDUCTION Robert W. Gundlach, Spencerport, N.Y.,assignor to Xerox Corporation, a corporation of New York Filed Mar. 17,1960, Ser. No. 15,704 15 Claims. (Cl. 317-262) This invention relates ingeneral to the field of electrostatics and in particular to thedeposition of charge in xerography.

In the art of xerography, as originally conceived, it has been usual toemploy a sensitive photoconductive insulating layer and expose it to apattern of activating radiation to form a developable electrostaticcharge pattern. This pattern has generally been developed on the layeritself requiring transfer of the developed image and generally cleaningof the photoconductive insulating layer prior to reuse. Techniques haverecently been developed whereby charges are deposited on anothersurface. These techniques are described, for example, in US. Patents2,221,776; 2,833,930; 2,833,648; 2,825,814; 2,912,586; 2,774,921;2,919,967 and co-pending applications U.S. Serial No. 651,979; 633,331;633,261; now US. Patents 2,934,650; 2,934,649 and 2,977,943,respectively, and 532,534. As is apparent from an examination of thisreference material, these techniques of charge deposition on surfacesare dependent on certain phenomena taking place between contiguousmembers or surfaces. For example, in US. Patent 2,221,776 the principleof electron emission is relied upon for charge flow to a contiguoussurface, and in US. Patent 2,825,814 a form of field stress is involvedto form image patterns and a similar stress is involved to uniformlycharge the surface in 2,833,930. Problems arise with these various priorart techniques. For example, when dealing with the concept ofphotoemission, it is difficult to find photoemissive layers whichmaintain their qualities while exposed to air, and with stress chargemigration one is faced with problems of critical spacings for practicaltype machines and avoiding foreign materials such as dust particles fromgaps in order to maintain uniform gap stresses to thus create uniformcharge deposits.

These and other difficulties are overcome by the instant invention inwhich charge is deposited in accordance with field controls and throughthe use of an insulating rectifying layer. A further accomplishment ofthe instant invention is its ability to operate in substantially anyfluid (liquid or gas) medium or in a vacuum.

It is accordingly an object of this invention to provide new improvedmethods of uniformly charging a surface.

It is another object of this invention to provide new and improvedmethods of selectively depositing electrostatic charge on a surface.

It is a further object of this invention to provide new and improvedapparatus to uniformly charge a surface.

It is a still further object of this invention to provide new andimproved apparatus to selectively deposit charge on a surface.

It is a still further object of this invention to devise novel means ofcharging in xerography.

It is a further object of this invention to devise novel and improvedapparatus for image formation in xerography.

Other objects and advantages of the present invention will be morereadily apparent in view of the following detailed description,especially when read in conjunction with the accompanying drawings,wherein:

FIG. 1 illustrates an embodiment of charging apparatus according to thisinvention;

FIG. 2 is another embodiment of charging apparatus according to thisinvention;

FIG. 3 is a still further embodiment of charging apparatus oralternatively image forming apparatus according to this invention;

FIG. 4 includes schematic diagrams of charge induction in accordancewith this invention; and,

FIG. 5 is an embodiment of an automatic machine according to thisinvention.

For a better understanding of this invention, reference is now had toFIG. 1 wherein is shown an embodiment of charging apparatus to place auniform charge across a surface. A chargeable member 10 is positioned onsupport means 11 which, in this embodiment, has edge guides 12 onsupport table 13. Positioned and disposed on support table 13 is motor15 which may, for example, be electrically operated from a -volt AC.power source. Extending outwardly and fixedly mounted to the housing ofmotor 15 is sleeve 17. Screw 16 which is directly connected to motor 15rotates freely in sleeve 17 when motor 15 is operating. Moving sleeve 18mounted on screw 16 includes a protruding pin or the like which extendsinto the helical path extending along screw 16. As screw 16 rotates, thepin protruding into the helical path in screw 16 imparts motion tosleeve 18. At the ends of screw 16 the path followed by the pin reversesthe direction of sleeve 18. Attached to sleeve 18 is charg ing roller20. The roller in this embodiment is spring mounted to allow positioningof chargeable members of variable thicknesses while maintaining roller20 in contact with the upper surface of chargeable member 10. Thephysical connection between sleeve 18 and roller 21) is electricallyinsulating to electrically isolate roller 20 from sleeve 18. Roller 20comprises an inner cylindrical electrode 21 of conductive material andan outer covering layer 22 of material having a resistivity generally inthe range of from 10 ohm-centimeters to 10 ohm-centimeters. Innerelectrode 21 of roller 20 is connected to power supply 23 and support11, which in this embodiment is conductive material, is electricallygrounded.

To operate the mechanism illustrated in FIG. 1, motor 15 is energizedcausing screw 16 to rotate thereby imparting motion to sleeve 18 causingroller 28 to roll across the upper surface of chargeable member 10 onsupport 11. In the embodiment illustrated, sleeve 18 travels along screw16 from left to right until at the right end of screw 16 sleeve 18changes its direction of motion and returns along screw 16 from right toleft. At the left end of screw 16, sleeve 18 contacts switch 25 whichstops operation of motor 15, which stops rotation of screw 16 andmovement of sleeve 18. Sleeve 18 thus remains at the left end of screw16 and in a position which will not interfere with the removal orinsertion of chargeable member 10 onto support 11. Further, in thisposition and with roller 20 out of the way, chargeable member 10 may beexposed to a pattern of activating radiation if a photoactive material.Roller 20 may be caused to move back and forth again over a chargeablemember by energizing motor 15 through a switch or the like. It is to berealized that other means of causing roller 26) to roll across andcreate a charging field through a chargeable member, generally known tothe art, are intended to be included herein and that the embodiment inFIG. 1 is for illustrative purposes only.

Reference is now had to FIG. 2 wherein another embodiment of a chargingdevice is illustrated. In this figure chargeable member 10 is beingmoved between two rollers 26 and 27. As in FIG. 1, each of the rollerscomprises an inner conductive electrode 21 surrounded by an outerresistive layer 22. As should be apparent, however, from FIG. 1, one ofthe rollers could readily be a pure conductor (as in support 11 in FIG.1). Rollers 26 and 27 are positioned within supports 28 and 30 and arepressed towards each other by springs 31. Electrodes 21 are electricallyinsulated from supports 28 and from each other by electricallyinsulating axle connections 32 and by chargeable memberin positionbetween the electrodes. Voltage is, in .this embodiment, supplied frompower supply 33 across resistance 35 having a point substantially in theelectrical center grounded and, asillustrated, the voltage across theends of the resistor is applied to the conductive portions 21 of rollers26 and 27. There is thus applied across chargeable member 10 a chargingvoltage to rollers 26 and 27. Rollers 26 and 27 are mounted on supports28 and 30 in such a manner as to freely rotate and chargeable member 10maybe moved manually therebetween, or means may be applied to themechanism of this figure such as a motor drive or the like to cause, asthrough friction, chargeablemcmber 10 to move throughjrollers 26 and 27and at a controlled and uniform rate of speed.

Reference is now had to FIG. 3 wherein another embodiment of a deviceaccording to this inventionis illustrated. The particular device shownin this figure is readily usable to uniformly charge a chargeablememberandmay be used to form image patterns of charge on the surface of achargeable member. As shall be apparent following consideration of imageformation in accordance with this figure, the devices of FIG. 1 and FIG.2 are also readily adapted to form image patterns on, as well as touniformly charge, a chargeable member. Considering now the device ofFIG. 3 functioning first as a charging device, a support layer 11 ofconductive material is attached to a pressure plate generally designated51 by hinges Pressure plate 51 comprises a conductive layer 52 and aresistive layer 53. An insulating connecting link 55 connects handle 56to pressure plate 51 allowing an operator to open and close pressureplate 51 away from and into contact with support layer Ill. Posh tionedbetween pressure plate 5'1 and support layer ll is chargeable member 10.Power supply 57 is electrically connected to conductive layer 52 ofpressure plate 51 and support layer Ill is grounded. To charge achargeable member in the device of FIG. 3, chargeablemember ll is placedon support layer 11 and pressure plate 51 is closed against the uppersurface of chargeable memher it). The potential is then applied by powersupply 57 creating a charging field through chargeable member lb. Thepotential may then be disconnected from con- I ductive layer 52 ofpressure plate 51 and pressure plate 7 51 raised upward so thatchargeable member is available to be removed from the device or to beexposed in position to an image pattern for subsequent utilization ifchargeable member 19 comprises a photoactive layer.

Although in the embodiments thus far described and illustrated achargeable member ltl has been illustrated as a single layer, it shouldbe appreciated that chargeable member 10 may comprise a chargeable layeroverlying a conductive backing layer. The conductive backing maycomprise alurninum, brass, paper, foil, or the like and when present,then all of the structures illustrated in FIGS. 1, 2 and 3 can bemodified to the extent that there is no need to support the chargeablemember on a conductive support layer. Instead, any support material maythe p-type when the majority charge carriers are holes 1 and n-type whenthe majority charge carriers are electrons.

The Hall constant gives the sign of the majority charge carrier. if thematerial has no preferential conductivity for either polarity of chargecarriers, it may be rendered either p-type or n-type by the inclusiontherein of specilied impurities termed doping whereby it is renderedeither p-type or n-type as is now well known in the transistor art.Thus, a material which is strongly p-type will conduct holes or positivecharges far more readily than electrons or negative charges. Such alayer is thus a rectifying layer when joined to a material having arelatively diiferent conductivity preference. For utility in the instantinvention wherein rectification is utilized to impart an electrostaticcharge to an insulating layer, photoconductive insulators which arethemselves strongly p-type or strongly n-type may be used-per se. As isnow well known in the art, a photoconductive insulating material is aninsulator in that it contains no intrinsic charge carriers, but ratherpossesses only those charge carriers injected therein from an externalsource orgenerated therein by the activity of activating radiation.Where the photoconductive insulating layer is insui'licientlyrectifying,i.e., conducts both polarities, equally or is only slightly p-type orn-type, rectification may be. supplied by forming a highlyrectifyingbarrier layer betweenthe photoconductive insulating layer andthe conductive backing which injects charge carriers into thephotoconductive insulator or alternatively as discussed above-the layermay be doped. As examples, zinc oxide is strongly n-type sothatmodification ofthe material eitherby doping or by the interpositionof a barrier layer between the zinc oxide and the conductive electrodeare unnecessary for utility herein. Vitreous selenium is p-type andcommercial xerographic plates on an aluminum backing may be used asdescribed herein. However, as a properly prepared selenium layer is onlyslightly p-type, the use of such unmodified layers results in moderatelylong charging times. such as cuprous oxide between the vitreousseleniumand the conductive backing, the charging time may be reduced to theorder of 1 or 2 seconds. Alternatively, the selenium may be doped toeither render it highly p-type or even to reverse the polarity of themajor charge carriers to render the material n-type. Thus, as disclosedin US. 2,863,768, when alloyed with arsenic trisulfide, selenium becomeshighly n-type while still being an effective photoconductive insulator.Tellurium is also a suitable additive to selenium to r nder a vitreousselenium layer n-type.

The selenium may also be rendered more highly p-type by the inclusion ofvery minor amounts of halogen. In the case of doping with a halogen,care must be taken to prevent the inclusion of too much impurity whichwould render the material too conductivefor use as a photoconductiveinsulator herein.

As should be apparent, substantially all known photoconductiveinsulating layers can be employed in connection with this invention,either intrinsically or as modifled according to the teaching herein,and included within this group, for example, are semi-conductivematerials cooled below the temperature necessary for thermal generationof charge carriers therein which thus act as insulators.

These various materials have been referred to as insulators in that eachelemental area acts as controlled by fields surrounding that elementalarea. Thus, no lateral flow takes place across the material. Thischarcteristic in the layer of chargeable member 10 makes chargeablemember it) useful, for example, for a subsequent exposure if theinsulating rectifying layer is photoactivc so that, once sensitized,chargeable member It) may be exposed, for example, to a light imagepattern to release charge in areas exposed to light and thus dissipatecharge in conformance with the light pattern leaving behind an imagepattern of charge conformingto the unexposed or dark areas of a lightand shadow pattern. This image pattern of charge can then be developedin accordance with known principles as, for example, as taught by theartof xerograp-hy.

By the addition of a rectifying barrier layer Similarly, the quantity ofconduction in each individual area as controlled by the fieldssurrounding each individual area makes the chargeable member useful inconnection with forming image patterns. Image formation in accordancewith this invention will be discussed below.

It is noted that insulating rectifying layers which are not lightsensitive such as insulating binder layers in which the binder is opaqueor photosensitive layers overcoated with an opaque layer or the like mayalso be used in this invention. Such layers are particularly valuablewhen a developable image pattern is directly induced into the layer.

T he device in FIG. 3 may also be used to form charge patterns on achargeable member. This is accomplished by positioning a selective fieldpattern on, for example, pressure plate 51 as by the utilization forpressure plate 51 of a xerograp-hic plate comprising, for example, aselenium layer overlying a conductive base bearing a charge pattern onthe selenium surface. The plate, in effect, comprises a conductive layer52 and a photoconductive insulating layer 53 such as selenium. On thesurface of photoconductive insulating layer 53 facing downward towardschargeable member 14) there would exist a charge pattern conforming toan image pattern to be reproduced. This pattern could have been formedon the surface using conventional xerographic techniques, through theselective deposition of charge or the selective dissipation of charge.Power supply 57 in this embodimen-t is then connected to apply groundpotential to conductive backing 52 of plate 51. Handle 56 is attached toplate 51 as described previously or may comprise an easily attachable ordetachable member. The plate, for example, is charged and exposed in anopen position and handle 56 is then used to move the plate into a closedposition across a chargeable member to create a charge pattern on thechargeable member. Support layer 11, as illustrated, is grounded andassuming a polarity of positive charges across the surface ofxerographic plate 51, there is formed on the surface of an insulatingrectifying layer (chargeable member negative charges in position inareas adjacent to areas of positive charge on the surface ofphotoconductive insulating layer 53. The pat-tern on insulatingrectifying layer 10 conforms in configuration to the pattern on thesurface of phoitoconductive insulating layer 53. After plate 51 isremoved, the pattern will remain in position on chargeable member 10 forsubsequent utilization. The elemental area control which exists ininsulating rectifying layers maintains the pattern in position andprevents its dissipation as through lateral conductivity or reverseflow. Thus, there is created following the application of selectivefields through insulating rectifying layer 10, a charge patternconforming in configuration to the field pattern applied and asdescribed above, the field pattern may be applied using a xerographicplate bearing a charge pattern. As should be apparent, rather than usinga xerographic plate as pressure plate 51, there may also be used aninsulating layer at layer 53 hearing selectively deposited charges whichwere previously deposited in any of many known ways. For example,charges can be deposited through a scanning system or through any one ofthe many sys terns described above in the patents cited as referencematerial as prior techniques applied in the xerographic art. Similarly,patterns of field can be effected through the use of conductive orresistive characters surrounded by insulating material and a potentialapplied from power supply 57 to conductive layer 52 of pressure plate51. The conductive or resistive characters or other pattern willselectively create fields to induce flow to the surface of chargeablemember 16 conforming in configuration of the patterns present in layer5'3.

It should also be appreciated that the techniques described inconnection with FIG. 3 may also be used in FIG. 1 or 2 by feeding animage inducing member and an insulating rectifying layer together intothe devices 5 and with the proper potentials applied an image patternwill form on chargeable member 11 Reference is now had to FIGS. 4-A and4-B wherein there is illustrated schematically charge induction ofelemental areas in accordance with the invention. In FIG. 4-A chargeinduction is illustrated in which the charge inducing element comprisesan insulating layer bearing charge and in FIG. 4-B charge inductionusing a roller or a platen at a fixed potential is illustrated.

Referring now to FIG. 4-A, precharged capacitor 60 comprises anelemental area of an insulating layer bearing charge and may comprisethe photoconductive insulating layer of a xerographic plate or maycomprise an insulating layer such as Mylar, cellophane, or the likehearing deposited charge. Variable capacitor 61 is representative of thevariable air gap or other fluid gap between the charge inducing memberrepresented by capacitor 66 and the insulating rectifying layergenerally designated 62 and comprising capacitor 63 and rectifier 65. Avariable capacitor 61 is included in this schematic in order toillustrate the bringing of the precharged elemental area of the chargeinducing member, illustrated as capacitor 66, to and away from thesurface of the insulating rectifying layer 62. As the air gap betweenthe precharged insulator, represented as capacitor 60 and insulatingrectifying layer 62 is decreased, the capacitance of capacitor 61 isincreased, and as the air gap becomes extremely small due to the placingof the inducing member, represented as capacitor 66 in normal physicalcontact with insulating capacitor 62, the capacitance of capacitor 61becomes relatively greater than either capacitor 60 or capacitor 63.This results in substantially all of the charge initially appearing onthe lower plate of capacitor 66 now appearing on the upper plate ofcapacitor 61. This in turn results in charging of the lower plate ofcapacitor 61 with an equal amount of charge of opposite polarity.Charging of the lower plate of capacitor 61 is accomplished in thisinvention due to the properties of insulating rectifying layer 62 whichallows charge flow in one direction and blocks charge flow in the otherdirection. Thus, in this instance the lower plate of capacitor 61becomes charged negatively through a flow of charge through rectifier 65and when the capacitance of capacitor 61 is varied to decrease thecapacitance by increasing the air gap between capacitor 60 andinsulating rectifying layer 62, the charge which appeared on the surfaceof the lower plate of capacitor 61 appears on the upper plate ofcapacitor 63. A balancing opposite charge appears on the lower plate ofcapacitor 63 attracted to this plate through ground. There thus resultsan elemental area of charge opposite to the charge on the inducingmember, represented by capacitor 60 in this figure, on the upper surfaceof the rectifying insulating layer 62.

In FIG. 4-B this same concept is illustrated through the use of a rolleror platen at a fixed potential. The roller or platen is illustrated asthe upper plate of capacitor 61 and is connected to battery 66 whichapplies charge across variable capacitor 61 and insulating rectifier 62.As the capacitance of variable capacitor 61 is increased through therolling of a roller across the surface of insulating rectifying layer 62which decreases the air gap as the roller rolls into contact, chargewill appear across variable capacitor 61 again due to the conductivityof rectifier 65 in one direction as the roller or the like is. separatedor rolled out of contact, the charge appearing on the lower plate ofcapacitor 61 will appear on the upper plate of capacitor 63 and abalancing oppo- 'sit'e charge will be attracted to the lower plate ofcapacitor 63 from ground. There thus results, according to thisembodiment, elemental charge induction on the surface of insulatingrectifying layer 62 because of its ability to allow charge flow in onedirection and its ability to prevent dissipation or retain charge oncedeposited on its surface.

As should be apparent in connection with FIGS. 4-A and 4-B, theseschematics explain the mechanism of uniform charging of an insulatingrectifying layer and similarly explain the mechanism of selectivecharging of an insulating rectifying layer. The controlling factor ineach instance comprises the element employed to induce charge to thesurface of the insulating rectifying layer. If the inducing elementcomprises a uniformly charged member such as a uniformly chargedinsulator or a battery potential supplied, for example through a platen,roller or conductive element, then a uniform opposite charge willdeposit across the surface of the insulating rectifying layer. If, onthe other hand, the inducing member comprises a surface bearing aselective charge pattern or a selective field pattern such as aninsulator bearing selectively deposited charges or a conductivecharacter or pin matrix or the like, the charges induced to the surfaceof the insulating rectifying layer will conform in configuration to thepattern of charges or the field pattern of the inducing member whilebeing opposite in polarity.

Reference is now had to FIG. 5 in which is illustrated an embodiment ofautomatic apparatus employing the instant invention. A plate, generallydesignated 67, comprises a photoconductive insulating layer 68 overlyinga conductive backing member 70. Plate 67, in accordance with thisinvention, includes an insulating rectifying layer and may comprise, forexample, a selenium layer overlying a copper oxide interlayer overlyinga conductive aluminum base. Such a plate member is a preferred plate ofthe p-type and the zinc oxide binder type plate known in xerography is apreferred n-type xerographic member. Plate 67 is driven by a motor (notshown) in the direction illustrated by the arrow and moves first througha charging station, generally designated 71, next through an exposurestation, generally designated 72, and next to image induction stations,generally designated '73, then to an erase station, generally designated75. Following erase station 75, plate 67 may be recycled again orrecycling can be automatically continuous. Alternatively, the erase,charging and exposure mechanism may be placed in an inoperativecondition and the apparatus run as a copier duplicator. Charging in thisdevice is accomplished through the use of a plurality of rollersdesginated 76, each connected to power supply 77 and each acting toinduce charge to the surface of photoconductive insulating layer 68 inaccordance with the description appearing above. In effect, theapplication of a negative potential to rollers 76 and power supply 77will induce, given a proper insulating rectifying layer such asselenium, a positive charge to the surface of the photoconductiveinsulating layer 68. Plate 67 next moves to exposure station 72 whereatan image pattern selectively activates portions of photoconductiveinsulating layer 68 to dissipate charge in accordance with the exposurepattern. This may be accomplished as through the use of contact lightexposure mechanism or projection exposure mechanism including reflectionor transmission of light from the copy to be reproduced or mayalternatively be accomplished through the use of an X-ray pattern orother activating radiation pattern. Following movement of plate 67through exposure station 72, there is in existence on the surface ofphotoconductive insulating layer 68 an image pattern of charge. Thispattern is next moved to induction stations 73 whereat insulatingrectifying layers 78 supplied from supply spools 80 are moved aroundrollers 81 which, as illustrated in this embodiment, are maintained atground potential.

Speed of movement of rectifying insulators 78 is synchronized' with thespeed of rotation of'plate 67 and charge patterns are induced on thesurface of rectifying insulators 78 contacting plate 67 as described,for example, in connection with FIG. 4. Rectifying insulating layers 78may in this embodiment comprise layers of zinc oxide in an insulatingbinder overlying a conductive backing such as foil or paper in aconductive condition t} or the like. With a positive charge pattern onthe surface of plate 67, negative charge patterns conforming inconfiguration to the image pattern on photoconductive insulating layer68 will be formed on the surfaces of recti: fying insulators 78 at thecharge induction stations 73. Following image induction while rectifyinginsulators 78 are in contact with the surface of photoconductiveinsulating layer as, rectifying insulators 78 are moved throughprocessing stations 82. At processing stations 82 the charge patterns onthe surfaces of rectifying insulators 78 may be developed and fused toform hard copy or may be otherwise utilized as, for example, throughscanning or the like to create electrical signals of the image patternfor utilization as through display on cathode ray tubes or the like.Following passage through processing stations 82 in this embodimentthere is illustrated feeding of rectifying insulators '78 to take-upspools 83. As is well known in the art, these webs may be cut intoproper lengths and stacked rather than fed to takeup spools as shown. Inthis embodiment there are illustrated two charge induction stations 73.However, it should be realized that in employing the concepts of theinstant invention the original image is in no way affected or in no waydeteriorated while charge is induced into an adjacent member.Accordingly, although only two stations are shown, any number ofstations may be positioned around a charge inducing member or a chargeinducing member may be moved into contact with an insulating rectifierto induce charge into such members substantially any number of times.Following movement of drum 67 through charge induction stations 73, thephotoconductive insulating layer 68 is moved through an erase stationwhereat the photoconductive insulating layer may be uniformlyilluminated with light to erase the charge pattern from its surface andthus prepare drum 67 for recycling. Alternatively, erasure may beomitted and drum 67 may be recycled immediately to charging station 71whereat rollers 76 will act to charge all uncharged areas to a level ofcharge to create a uniformly charged plate. It should be appreciated asdiscussed in connection with FIG. 4 that the potential applied throughrollers 76 creates a charging level which is eventually obtained duringthe charging step. Accordingly, if a portion of the surface to becharged is originally charged passing the entire photoconductor throughthe charging station will result in placing a uniform charge across theentire surface. Plate 67 is then ready for exposure for furtherutilization. Positioned about the device illustrated in this figure is alight tight hood or cabinet $5. In this embodiment plate 67 has beendescribed as a light sensitive member and as such will require shieldingfrom light during passage through the various stations as illustrated inthis figure.

In all embodiments of the instant invention including both charginguniformly and charging selectively, two factors must be taken intoaccount. First, it is essential that there be present on the chargeinducing member that charge density which is desired on the surface ofthe insulating rectifier. If the equivalent of a battery as shown inFIG. 4-3 is being employed, then the ratio of the voltage applied by thebattery to that desired on the insulating rectifier is the ratio of thecapacitance per unit area of the insulating rectifying layer to thecapacitance per unit area of the minute air gap separating, the chargeinducing member and the insulating: rectifier. Thus, in general, thecapacitance of the air gap is much higher than the capacitance of theinsulating rectifier and the potential applied by an equivalent of abattery as shown in FIG. 4-B would be less than the desired potential onthe surface of the insulating rectifier. If, on the other hand, one isdepositing charge in accordance with the circuit illustrated in FIG.4-A, the capacitance of the ratio of the voltage on the charge inducingmember to the voltage induced on the insulating rectifier is the ratioof the capacitance per unit area of the insulating rectifier to thecapacitance per unit area of the charge inducing member. This latterexample, of course, is particularly applicable in the case of inducingimage patterns from an image pattern bearing insulator and the formerexample is, of course, particularly applicable to uniformly chargingthrough the use of either a roller, a platen, or forming an imagepattern using a shaped character to which a potential is applied, or thelike. Also pertinent to the applied voltage in terms of creating acharge on the surface of the insulating rectifier is an upper limit toprevent a breakdown which is fully explored in copending U.S. patentapplication Serial No. 718,247, now abandoned. This upper limit isgenerally no problem when considering the art of xerography inaccordance with known charging or image forming techniques as applied inconnection with this invention.

The second factor is concerned with the time necessary for charge to beinduced to the surface of an insulating rectifier. The time required forthe surface to reach a potential within 1/ e of its final value is theproduct of the resistivity of the material times the dielectric constantof the material both being expressed in MKS units. The resistivity, ofcourse, is that measured in the conducting direction. Because of thisfactor, it is necessary to either maintain contact for an adequate timeto create a situation of full charge or to provide a greater chargedensity in the charge inducing member and allow charge to be induced forless time as required for equilibrium to result in a charge deposit onthe insulating rectifier equal to what is desired.

There are now included a few examples of operation in accordance withthis invention.

Example 1 A commercial xerographic plate comprising a layer of vitreousselenium overlying an aluminum backing was uniformly charged in acommercial charging unit producing a charge potential of approximately600 volts on the surface of the selenium layer. A commercial xerographicplate comprising zinc oxide in an insulating silicone binder overlying apaper backing was placed, in the dark, across and in contact with theselenium xerographic plate with the zinc oxide binder layer contactingthe selenium layer. The zinc oxide plate was kept in contact for varioustimes between seconds and 60 seconds and was then removed in the darkand then exposed to a light pattern of an image for usual time. The zincoxide plate was then developed in the usual way using a magnetic brush.There appeared on the surface of the zinc oxide plate an imageconforming in configuration to the light image pattern.

Example 2 A commercial xerographic plate comprising a vitreous seleniumlayer overlying an aluminum backing member was charged and exposed inthe normal manner to form an image pattern of charge on its surface. Acommercial xerographic plate comprising zinc oxide in an insulatingbinder of silicone backed by paper was placed across the image bearingselenium plate with the zinc oxide binder layer in contact with theselenium layer. The zinc oxide binder plate was kept in contact forvarious times between 1 second and 60 seconds and was then removed anddeveloped in the usual way with a magnetic brush, thus making visible onthe Zinc oxide binder layer the original light image pattern to whichthe selenium plate has been exposed.

Example 3 The steps described in Example 2 were carried out andfollowing the positioning and separation of a zinc oxide plate acrossthe selenium plate surface, a second zinc oxide plate similar to the oneused in the first manipulation was placed across the selenium surfacewith the zinc oxide layer in contact with the selenium layer and thesecond zinc oxide binder plate was then removed and developed forming avisible reproduction which duplicated that produced in Example 2. Thesame manipulation was carried out with a third and fourth zinc oxidebinder plate producing duplicate developed images in each instance.

Example 4 A commercial xerographic plate comprising zinc oxide in asilicone binder overlying a paper backing was charged in a commercialcharging unit and was placed with the zinc oxide binder layer in contactwith a selenium layer of a xerographic plate comprising a selenium layeroverlying an aluminum layer with an interface of copper oxide betweenthe aluminum layer and the selenium layer. The selenium plate wasmaintained in this position for various time periods between 5 secondsand 60 seconds and was then removed and exposed and developed in thenormal manner producing a developed image on the selenium layerconforming in configuration to the light image pattern.

Example 5 The same steps were carried out as described in connectionwith Examples 2 and 3 using as the master or inducing member a zincoxide binder plate and using as the insulating rectifying layer theselenium layer plate structure including a copper oxide interlayer asdescribed in Example 4, and developed images were produced on theselenium surface. The times varied between 1 second and 60 seconds.

Example 6 A roller comprising an inner conductive core and an outerresistive coating having a resistivity of about 10" ohm-centimeters, theresistive coating being about an inch thick, was rolled across thesurface of a xerographic plate comprising a photoconductive insulatinglayer of vitreous selenium overlying a conductive layer of aluminumhaving an interface between the aluminum and selenium layer of copperoxide. Potentials between the range of -50 volts and 500 volts wereapplied to the conductive core of the cylindrical roller and the rollerwas moved at a substantially uniform rate of speed across the surface ofthe photoconductive insulating layer in darkness while the aluminumbacking of the plate was grounded. The rate of movement of the rolleracross the plate was between about /2 and 1 inch per second. The platewas then, while kept in darkness, moved to an exposure position andexposed to a light image pattern and then developed. Best results wereobtained using potentials between 200 to 350 volts.

The same experiment was carried out in connection with a xerographicplate in which the photoconductive insulating layer comprising zincoxide in an insulating .binder of a silicone resin. The voltage range inthis instance was between 50 volts and 500 volts. The plate was thenexposed and developed in a normal manner to produce a visible image ofthe light image pattern.

In the case of the selenium photoconductive insulating layer, thevoltage applied to the cylindrical core was negative in polarity and thedeveloper material included negatively charged toner, and the imagedeveloped included toner depositions in areas of shadow in the light andshadow pattern to which the plate was exposed. In the case of the zincoxide insulating binder photoconductive layer, the core of the rollerwas connected to a positive potential and the developer toner waspositively charged and the developed image included developer materialdeposited in areas not exposed or in shadow areas of the light andshadow pattern to which the image pattern was exposed.

As should be apparent, charging in accordance with this invention isdependent on field applied through a material which conducts in onedirection and blocks in '11 the reverse direction. Accordingly, theinducing member could comprise a conductive layer positioned closelyadjacent to the surface of the insulating rectifying layer. However,since defects in the surface of the insulating rectifying layer wouldresult in av discharge which would .remove substantially the field fromother areas, it is desired to use in a platen, as for example, shown inFIG. 3, or on rollers, as for example shown in FIGS. 1 and 2, aresistive layer including a material having a resistivity above about 10ohm-centimeters in order to prevent rapid charge flow through defectsand to continue a uniform field acrossthe insulating rectifying layer.

Also, as should be apparent in connection with the examples above,variations in capacitance will result in a variation in the appliedpotential or the created charge as previously discussed.

There have been given a number of examples of opoperation in accordancewith this invention. These examples, plus the discussion of theinvention in connection with the drawings, will suggest obviousmodifications and variations to those skilled in the art, and suchvariations and modifications are intended to be included within thescope of this invention, and his intended to cover the invention broadlywithin the scope of the appended claims.

I claim:

1. The method'of charging aplate including an in sulating rectifyinglayer capable of allowing flow of current in a first direction andsufficiently insulating'in the reverse direction to retain charge on asurface thereof comprising, applying to said plate a fielclhaving adirection to cause current flow in said first direction, and thenremoving the applied fieldtrapping charge on the surface of saidrectifying insulating layer thereby producing a field across said plateopposite in direction to the applied field.

'2. The method of uniformly charging a plate including an insulatingrectifying layer capable of allowing fiow of current in a firstdirection and sufficiently insulating in the reverse direction to retaincharge on a surface thereof comprising, in the absence of light,applying to said plate a uniform field having a direction to cause.current flow in said first direction, and then removing the appliedfield trapping charge on a surface of said insulating rectifying layerthereby producing a field across said plate opposite in direction to theapplied field.

3. The method of claim 2 in which said plate includes a layer ofvitreous selenium. and in which the field is applied to cause depositionof positive polarity charges across an external surface of said vitreousselenium layer.

4. The method of claim 2 iniwhich said plate includes a layer of zincoxide in an insulating binder and in which said field is applied tocause deposition of negative polarity charges across an external surfaceof the zinc oxide binder layer.

5. The method of claim 2 in which said uniform field is applied byrolling a roller electrode at a substantially uniform speed across theplate member.

6;The method of claim 2 in which said uniform field is applied bypositioning a platen electrode across the area of the plate to becharged and by maintaining said electrode in position for a sufficientperiod of time to create a desired amount of charge on the plate member.

7. The method of claim 2 in which said uniform field is applied bypositioning a charged insulating layer across asurface ofsaid plate andby maintaining the insulating layer in position for a sufficient timeperiod to create a desired amount of charge on the plate member.

' 8. The method of'claim 7 in which said insulating layer comprises thephotoconductive insulating layer of a xerographic plate which has beenuniformly charged with corona charge.

9. The method of selectively depositing chargeon the 4 surface of aplate including an insulating rectifying layer capable of allowing flowof current in a first direction and sufficiently insulating in thereverse direction to retain charge on a surface thereof comprising,applying to said plate an electrostatic field in image configurationhaving a direction to cause current fiow in said first. direction,

and then removing the applied field trapping charge in imageconfiguration on a surface of said insulating rectifying layer therebyproducing a field across said plate opposite in direction to the appliedfield.

10. The method of claim 9 in which said plate include a layer ofvitreousselenium and in which the field is applied to create an imagepattern of positive polarity charges on an external surface of thevitreous selenium layer.

11. The method of claim 9 in which said plate includes a layer of zincoxide in an insulating binder and in which said field is applied tocreatean image pattern of negative poiarity charges on an'externalsurface of the zinc oxide binder layer.

12. The method of claim 9 in which the field in image pattern is appliedby positioning across the surface of said plate an insulating layerbearing charges in image configuration.

13. The method of claim 9 in which the field in image pattern is appliedby. applying a potential .to an electrode member comprising areas ofconductive and insulating material varying in accordance with an imagepattern to be reproduced.

14. The method'of claim 11 in which said insulating layer comprises thephotoconductive insulating layer of a xerographic plate having an imagethereon formed by sensitizing said xerographie plate and exposing saidxerographic plate to an image pattern.

15. The method of depositing charge on a layer mem-- her at leastcapable of current flow in a first direction comprising positioning saidmember in an electrostatic :field to cause current flow in said firstdirection through said member and in the absence of light exposure ofsaid member, and then trapping charge on said member while said memberis sufiiciently insulating in the reverse direction to said firstdirection and removing said applied field from said member therebyproducing across said layer member a field, opposite in direction tosaid applied field.

References Cited inthe; file of this patent UNITED STATES PATENTS

9. THE METHOD OF SELECTIVELY DEPOSITING CHARGE ON THE SURFACE OF A PLATEINCLUDING AN INSULATING RECTIFYING LAYER CAPABLE OF ALLOWING FLOW OFCURRENT IN A FIRST DIRECTION AND SUFFICIENTLY INSULATING IN THE REVERSEDIRECTION TO RETAIN CHARGE ON A SURFACE THEREOF COMPRISING, APPLYING TOSAID PLATE AN ELECTROSTATIC FIELD IN IMAGE CONFIGURATION HAVING ADIRECTION TO CAUSE CURRENT FLOW IN SAID FIRST DIRECTION, AND THENREMOVING THE APPLIED FIELD TRAPPING CHARGE IN IMAGE CONFIGURATION ON ASURFACE OF SAID INSULATING RECTIFYING LAYER THEREBY PRODUCING A FIELDACROSS SAID PLATE OPPOSITE IN DIRECTION TO THE APPLIED FIELD.